THIS invention relates to a mammographic tomography test object (hereinafter referred to as a phantom) and more particularly, but not exclusively, to a mammographic tomography test phantom for use in the verification of image reconstruction positions in digital breast tomosynthesis. The invention also relates to a method of verifying image reconstruction positions in digital breast tomosynthesis using a customized mammographic tomography test phantom.
Breast cancer is one of the most common cancers affecting women all over the world. Early detection is vital so that the patient can be diagnosed and treated, as the chance of survival is much better when breast cancer is detected at an early stage. Over the past few years advances in technology have ensured an increase in the development and uptake of mammography systems.
In standard two dimensional (2D) projection film screen mammography (FSM) or full field digital mammography (FFDM), overlapping dense fibroglandular tissue within the breast can reduce the visibility of malignant abnormalities or can simulate the appearance of an abnormality. This can lead to unnecessary recalls, biopsies and psychological stress for the women concerned. In this conventional mammography, overlapping or superimposed tissue creates a clutter of signals above and below objects of interest, which can for example result in lesions being hidden by other objects.
Digital breast tomosynthesis (DBT) is a relatively newly developed form of three dimensional (3D) image reconstruction which has the potential of improving the accuracy of mammography by blurring out overlying tissues. It is based on the outdated linear tomographic technique (superseded by Computed Tomography (CT) scanning) that was used in the 1970s to achieve an equivalent goal in general x-ray imaging. The overlying tissue in the image, sometimes referred to as anatomical noise, degrades image quality in standard 2D projection imaging. The definition of tomosynthesis, as stated in the LORAD Hologic Selenia Dimensions Quality Control manual is “an imaging technique that recombines a number of breast images taken at different angles through the breast to achieve various effects”. In DBT, multiple projection images of the breast (approximately 15) are acquired from different angulations of the x-ray tube within a single plane of motion perpendicular to the detector and in line with the front edge of the detector. The images are then processed using reconstruction algorithms to produce tomographic sections through the breast. These sections can be viewed on a computer as planes or slices. By reducing the superimposition of overlying breast tissue, DBT has the potential to differentiate malignant features more definitively from non-malignant ones. It should be noted that the improvements to linear tomography (originally developed in the 70's) included non-linear translations which increased out of reconstruction plane image blur thus improving in reconstruction plane resolution. These non-linear translations included circular and hyper-cycloidal movements.
It is obvious that, as with any other form of radiology, in DBT image quality is very important and should be optimized. Various parameters influence the image quality, two of which are the angular range of the x-ray tube, and the number of exposures. These parameters are usually fixed on any particular DBT system, but have a huge impact on image quality
The larger the angular range, the wider the separation of the slices and increase in the out of plane resolution. However, with smaller angles more structures will be in focus in a specific plane. More angles at which images are acquired will result in more exposures which would reduce the visibility of artefacts, but more ionising radiation dose will be delivered to the patients. Compensation by reducing the dose per view is limited as this will introduce more statistical noise in the image and may at some point start to deteriorate image quality.
An artefact, in the radiological sense, refers to any perceived structure that is not actually present, but is produced by the imaging process, i.e. something not anatomically real such as the distortion of a structure or signal, which interferes with or obscures the interpretation of a study, or a structure that is not representative of a specimen's in vivo state and which does not reflect the original sample, but rather is the result of an imaging procedure, its analysis or other factors. The smearing/out of plane artefact is one of the most prominent artefacts found in DBT. These artefacts may thus lead to uncertainties in where the planes are in DBT, as well as how accurately the reconstruction plane can be said to be a fixed thickness, such as 1 mm slice thickness. Therefore, for the image quality to be optimized the image reconstruction plane position and characteristics need to be defined. This uncertainty should be quantified as part of the quality program for DBT systems, to determine accurately the actual image reconstruction planes (IRP) obtained during tomographic reconstruction.
It is accordingly an object of the invention to provide a mammographic tomography test phantom for use in the verification of image reconstruction depths in digital breast tomosynthesis, which will enable the accurate determination of image reconstruction planes.
It is also an object of the invention to provide a method of verifying image resolution at various positions in digital breast tomosynthesis images by customizing the mammographic tomosynthesis test phantom, and developing an appropriate analysis algorithm, which method will enable the quantification of image resolution degradation at various points and in two planes in the image (in the plane of motion and perpendicular to the plane of motion of the DBT x-ray source).